A light emitting diode (LED) is a sort of a semiconductor device which converts electricity into infrared or visible light using characteristics of a compound semiconductor to exchange signals, and is used as a light source.
Group III-V nitride semiconductors have been spotlighted as a core material of a light emitting element, such as a light emitting diode (LED) or a laser diode (LD), due to physical and chemical characteristics thereof.
Since such light emitting diodes do not contain environmentally harmful materials such as mercury (Hg) used for existing lighting apparatuses, such as incandescent lamps or fluorescent lamps, and have long lifespan and low power consumption, the light emitting diodes are a replacement for existing light sources.
In general, a light emitting module includes a light emitting element package and the light emitting element package includes a light emitting element such as an LED.
FIG. 1 is a diagram showing the waveform of a ripple voltage obtained by full-wave rectifying an alternating current (AC) voltage in an existing light emitting module, wherein V denotes voltage and I denotes current.
In general, if an LED is used as a lighting apparatus, a plurality of LEDs is connected in series or in parallel and the LEDs are turned on or off by a driving integrated circuit (IC) of the light emitting module. The driving IC for controlling the plurality of LEDs generally rectifies an AC driving voltage and sequentially turns the plurality of LEDs on or off according to level change of the rectified ripple voltage V. At this time, by changing a time for applying the driving voltage and the level of the driving voltage, total harmonic distortion (THD) and power factor (PF) of the lighting apparatus may be determined. Referring to the waveform of FIG. 1, the LED is repeatedly turned on and off due to the characteristics of the ripple voltage V. That is, in the periods of the full-wave-rectified ripple voltage V, current I having a predetermined pattern is continuously supplied to turn the LED on in a period in which the ripple voltage V is equal to or greater than a predetermined voltage, and current I is not supplied to turn the LED off in a period in which the ripple voltage V is less than the predetermined value.
As described above, since the existing light emitting module controls ON or OFF of the LED with a very short period, flicker inevitably occurs. Although flicker is not easily visually recognized to the naked eye, when a person is exposed to flicker for a long period of time, the flicker may aggravate the person or cause the person to easily feel fatigued.
FIG. 2 is a graph illustrating a general flicker index, wherein a horizontal axis denotes a time and a vertical axis denotes light output.
A degree of flicker may be expressed by the flicker index. Referring to FIG. 2, the flicker index is expressed by Equation 1 below.
                              A          ⁢                                          ⁢          1                                      A            ⁢                                                  ⁢            1                    +                      A            ⁢                                                  ⁢            2                                              Equation        ⁢                                  ⁢        1            
where, referring to FIG. 2, A1 denotes an upper area of a portion having a value greater than an average light output AV and A2 denotes a lower area of a portion having a value less than the average light output AV. That is, the flicker index may be expressed by a ratio of the upper area to the total area. Such a flicker index has a value of “0” to “1”.
Referring to FIGS. 1 and 2, as the level difference of current I increases, the flicker index increases and thus needs to be improved. In addition, the light emitting module needs to be designed with a high withstand voltage.